1. Field Of The Invention
This invention relates to electrographic writing systems and in particular to a device for generating an electric field of maximized intensity between a conductive transport belt and a recording medium supported thereon to thereby provide a simplified means for adhering and stabilizing a recording medium on a transport belt in a multipass, raster line printer, plotter or the like. The electric field paper stabilizing system according to the present invention employs a continuous, conductive transport belt having a changing radius of curvature for supporting a recording medium in substantially a complete revolution past a writing head and toner assembly. The transport belt and recording medium act as parallel conductive plates of a capacitor which are separated by a thin dielectric coating provided on the paper holding surface of the transport belt.
In an electrographic writing system such as an electrostatic plotter or printer, it is crucial to stabilize the paper with an extreme degree of precision with respect to a writing head and toner assembly so that the paper can be plotted on multiple writing passes. It is also necessary to keep a recording medium such as dielectric paper immovably stabilized during plotting operations in order to obtain high resolution and an improved degree of color to color registration.
Prior art methods for holding a sheet of paper to a transport belt in a plotter, printer or the like often teach the application of a vacuum to a recording medium by a vacuum plenum or vacuum generating means disposed on the underside of a porous or perforated belt. Examples of such prior art methods are shown in Gablin, et al., U.S. Pat. No. 4,207,579 or Goel, U.S. Pat. No. 4,023,894.
These methods have notable disadvantages. Because vacuum is applied to the paper only when the porous belt is positioned directly over a vacuum plenum, once the belt moves off the plenum the paper cannot be stabilized on the belt because the vacuum instantly escapes through the perforations or pores in the belt. The foregoing methods are inapplicable in a high speed plotter in which paper must be adhered to a continuous belt which makes multiple passes past a writing head because these methods of paper stabilization are unable to hold paper on a continuous transport belt as the belt moves over rollers.
The typical prior art vacuum hold down system is also mechanically extremely complex. Such a vacuum system requires a vacuum plenum or other means of producing a vacuum, vacuum pumps, vacuum hoses and flexible seals which must travel with the belt in order to achieve a continuous application of the vacuum to the recording medium. It is difficult to completely stabilize a recording medium on a transport belt in a plotter or printer to the need for a flexible, moving vacuum seal or other means for continually applying the vacuum to the underside of the paper as it travels on the belt. Because such flexible vacuum seals are continuously moving, they are subject to constant wear and are particularly susceptible to leaks, especially when the belt must travel at high speed.
Some prior art methods apply a vacuum to a recording medium by means of vacuum channels on a rotating belt. The vacuum is applied to the channels by hoses or a vacuum plenum which must rotate with the belt. It would be desirable to eliminate vacuum pumps, traveling vacuum hoses and flexible vacuum seals which are susceptible to leaking and which add undue mechanical complexity to a plotter or printer incorporating a moving belt.
Paper stabilizing systems are known which use electrostatic attraction for holding the paper to a plotting surface. U.S. Pat. No. 3,571,678 is an example of a prior art paper stabilizing apparatus for a flat bed pen plotter, recording drum or other supporting structure having a uniform surface. This prior art device includes a base comprising an insulating sheet. The insulating sheet has a thin metallic layer on its lower side which is adhesively supported on the supporting structure. The metallic layer forms an electrode. A semiconductor layer is disposed on the upper side of the insulating sheet. The top surface of the semiconductor layer further supports a conductive pattern comprising strips of conductive material such as metallic foil, sheet metal or even conductive paint. A rectangular edge frame of sheet metal is superimposed above the conductive pattern for applying a uniform voltage to all portions of the conductive pattern. A voltage potential is applied between the lower metallic layer and the frame. Paper disposed above the frame and conductive pattern is held on to the conductive pattern by electrostatic attraction. This method is somewhat complex and requires a semiconductor layer in order to discharge any frictional charge which may be generated due to friction as the paper is moved from the holding device. The semiconductor layer was also thought to be necessary in order to prevent any frictional charge from interfering with or impeding the paper attracting action.
The foregoing prior art method for using electrostatic attraction to hold paper to a rigid drum or bed is not believed adaptable to a modern high speed plotting system utilizing a continuous transport belt. Moreover, it would not be obvious to use such a system in an electrostatic plotter because the generated electrostatic field for holding the paper would interfere with the electrostatic writing field developed from the writing nibs. Therefore, what is needed is a simplified paper stabilizing system characterized by high reliability which would not be subject to loss of registration. Such a paper stabilizing system should be adaptable for stabilizing a recording medium in a high speed, high resolution electrostatic plotter. Such a system should desirably eliminate the mechanical complexity and unreliability of flexible, moving vacuum seals and vacuum hoses of prior art vacuum paper hold down systems.
The paper stabilizing system should be capable of stabilizing a recording medium against even minute dimensional and positional changes due to temperature and humidity. It is preferable that such a paper stabilizing system be incorporated in a plotter having a continuous transport belt configurable as an ellipsoid with a changing radius of curvature for passing the recording medium a complete revolution past a writing head and toning assembly on multiple writing passes. Such a system would maximize plotting speed and at the same time, maximize the surface area of the recording medium capable of being written upon while maintaining a highly compact plotter size.